For registering process-describing, measured variables of flowing media, such as the thermodynamic state variable, density, or measured variables derived therefrom, and for producing measured-values correspondingly representing such measured variables, industrial process measurements technology applies measuring systems installed near to the process. This is done especially also in connection with the automation of chemical processes or processes involving adding value to materials. These measuring systems are often composed of two or more, discrete, measuring, field devices, which communicate with one another and are each arranged directly at, on or in a process line, through which the medium flows. The measured variables to be registered can include, besides density, for example also other thermodynamic state variables, especially such variables as are registerable by sensor and, as a result, directly measurable, such as e.g. pressure or temperature, directly or indirectly measurable flow parameters, such as e.g. a flow velocity, a volume flow, e.g. a volume flow rate, or a mass flow, e.g. a mass flow rate, or other complex transport variables, such as e.g. a heat flux, as well as also other measured variables specific to the medium, such as e.g. a viscosity, of an at least partly liquid, powdered or gaseous medium conveyed in a process line embodied, for example, in the form of a pipeline.
Especially for the indirect (in the following, thus, also referred to as virtual) measurement of density, based on pressure and temperature measurement signals generated by means of corresponding sensors, as well as also measured variables possibly derived therefrom, for example mass flow or volume flow, a large number of industrial standards have become established, which recommend a largely standardized, and, thus, comparable, calculation, especially also with application of directly registered and, thus, actually measured temperatures and/or pressures, and which find application as a function of application area and medium. Examples of such standards include, by way of example, the industrial standard “IAWPS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam”, International Association for the Properties of Water and Steam (IAWPS-IF97), “A.G.A. Manual for the Determination of Supercompressibility Factors for Natural Gas—PAR Research Project NX-19”, American Gas Association (AGA-NX19, Library of Congress No. 63-23358), the international standard ISO 12213:2006, Part 1-3 “Natural gas—Calculation of compression factor”, as well as also the therein cited A.G.A. Compressibility Factors for Natural Gas and Other Related Hydrocarbon Gases”, American Gas Association Transmission Measurement Committee Report No. 8 (AGA-8) and “High Accuracy Compressibility Factor Calculation for Natural Gases and Similar Mixtures by Use of a Truncated Viral Equation”, GERG Technical Monograph TM2 1998 & Fortschritt-Berichte VDI (Progress Reports of the Association of German Engineers), Series 6, No. 231 1989 (SGERG-88).
Often, the ascertaining of density can also serve for converting a directly measured, mass flow into an, as a result, indirectly or virtually measured, volume flow, or vice versa. For direct measurement of flow parameters serving as primary measured variables therefor—thus, for example, a local flow velocity, a local volume flow, or a local mass flow—measuring systems of the type being discussed include at least one corresponding flow sensor, which, by reacting at least predominantly to a flow parameter primarily to be registered for the flowing medium, or also to changes of the same, delivers, during operation, at least one measurement signal, especially an electrical measurement signal, correspondingly influenced by the measured variable primarily to be registered and representing such as accurately as possible. The at least one flow sensor can, in such case, be embodied to contact the medium, at least partially, for example by being immersed therein, or to measure externally through the wall of the process line or a membrane, or diaphragm. Usually, the flow sensor is provided, in such case, by means of a, most often, very complex flow transducer, which is inserted appropriately directly into the process line, or into a bypass, conveying the medium.
Marketed flow transducers are usually implemented as prefabricated, pre-calibrated units equipped with a carrier tube insertable into the course of the pertinent process line and also with at least one physical-to-electrical converting element appropriately pre-assembled therewith. This converting element, possibly in conjunction with the carrier tube itself and/or other components of the flow transducer, especially passive-invasive components, such as e.g. flow obstacles protruding into the flow and/or active components of the flow transducer, such as e.g. a coil arrangement placed externally on the support tube for generating a magnetic field, or sound producing units, forms the at least one flow sensor delivering the measurement signal. Widely distributed in industrial measurements technology are, especially, magneto-inductive flow transducers, flow transducers evaluating the travel time of ultrasonic waves coupled into flowing media, eddy flow transducers, especially vortex flow transducers, flow transducers with oscillating measuring tubes, flow transducers making use of pressure differences, or thermal flow-measuring transducers. Principles of construction and functioning of magneto-inductive flow transducers are described e.g. in EP-A 1 039 269, U.S. Pat. Nos. 6,031,740, 5,540,103, 5,351,554, 4,563,904, while such for ultrasonic flow transducers appear e.g. in U.S. Pat. Nos. 6,397,683, 6,330,831, 6,293,156, 6,189,389, 5,531,124, 5,463,905, 5,131,279, 4,787,252. Since also the others of the aforementioned principles of measurement usually put into practice in industrial flow measuring transducers are likewise sufficiently known to those skilled in the art, a further explanation of these and other principles of measurement established in industrial measurements technology and implemented by means of flow measuring transducers can be omitted here.
Industrial measuring systems registering flow parameters involve, often, those, in the case of which, at least one of the measuring points delivering the actual measurement signals and thus, in the following, referred to as real, is formed by means of a compact inline measuring device having a flow transducer of the aforementioned kind. Further examples for such measuring systems, especially measuring systems formed by means of compact, inline measuring devices with flow transducers known per se to those skilled in the art, are presented, additionally, in detail in, among others, EP-A 605 944, EP-A 984 248, EP-A 1 767 908, GB-A 21 42 725, U.S. Pat. Nos. 4,308,754, 4,420,983, 4,468,971, 4,524,610, 4,716,770, 4,768,384, 5,052,229, 5,052,230, 5,131,279, 5,231,884, 5,359,881, 5,458,005, 5,469,748, 5,687,100, 5,796,011, 5,808,209, 6,003,384, 6,053,054, 6,006,609, 6,352,000, 6,397,683, 6,513,393, 6,644,132, 6,651,513, 6,651,512, 6,880,410, 6,910,387, 6,938,496, 6,988,418, 7,007,556, 7,010,366, US-A 2002/0096208, US-A 2004/0255695, US-A 2005/0092101, US-A 2006/0266127, WO-A 88/02 476, WO-A 88/02 853, WO-A 95/08758, WO-A 95/16 897, WO-A 97/25595, WO-A 97/46851, WO-A 98/43051, WO-A 00/36 379, WO-A 00/14 485, WO-A 01/02816, WO-A 02/086 426, WO-A 04/023081 or WO-A 04/081500, WO-A 05/095902, as well as also in the not pre-published applications DE 102006034296.8 and 102006047815.0 of the assignee.
For the further processing or evaluation of measurement signals produced in the measuring systems, such additionally include at least one corresponding measuring electronics. The measuring electronics, communicating in suitable manner with the pertinent measuring transducer, especially also with the at least one converting element, produces during operation, with application of the at least one measurement signal, repeatedly, at least one measured-value instantaneously representing the measured variable, thus, for example, a mass flow measured-value, volume flow measured-value, a density measured-value, a viscosity measured-value, a pressure measured-value, a temperature measured-value, or the like. The measured-values, especially the indirectly, or also virtually, measured, density measured-value, are, in such case, often ascertained by means of highly complex calculations according to one of the mentioned industry standards, for example “AGA 4”, “AGA 8”, “AGA-NX19, “IAWPS-IF97”, “SGERG-88”, or the like.
For accommodating the measuring electronics, such measuring systems include, most often, a corresponding electronics housing, which, as proposed e.g. in U.S. Pat. No. 6,397,683 or WO-A 00/36 379, can be arranged remotely from the measuring transducer and connected with such via a flexible cable. Alternatively thereto or in supplementation thereof, the electronics housing can, however, also, as shown, for example, in EP-A 903 651 or EP-A 1 008 836, be arranged directly on the measuring transducer or on a measuring transducer housing separately housing the measuring transducer, in order to form a compact, inline measuring device, for example a Coriolis mass flow/density measuring device, an ultrasonic flow-measuring device, a vortex flow-measuring device, a thermal flow-measuring device, a magneto-inductive flow-measuring device, or the like. In the case in which the electronics housing is arranged on a measuring transducer housing, the electronics housing serves, as shown, for example, in EP-A 984 248, U.S. Pat. Nos. 4,716,770 or 6,352,000, often also for accommodating some mechanical components of the measuring transducer, such as e.g. elements deforming during operation on the basis of mechanical effects, elements such as membrane, rod, sleeve or tubular deformation- or vibration-elements; compare, in this connection, also the U.S. Pat. No. 6,352,000 mentioned above.
In the case of measuring systems of the described kind, the measuring electronics is usually electrically connected via electrical lines, and/or wirelessly by radio, with a superordinated, electronic, data processing system arranged, most often, spatially remotely, and also spatially distributed, from the measuring electronics. To this data processing system are forwarded, in near-time, the measured-values produced by the measuring system. The measured-values are forwarded by means of measured-value signals carrying the measured-values. Measuring systems of the described kind are, additionally, usually, by means of a data transmission network (wired- and/or radio-based) provided within the superordinated data processing system, connected together and/or with corresponding electronic process controls, for example programmable logic controllers (PLCs) installed on-site or process control computers installed in a remote control room, where the measured-values produced by means of the measuring system and digitized in suitable manner and correspondingly encoded are sent. By means of process control computers, with application of correspondingly installed software components, the transmitted measured-values can be further processed and visualized as corresponding measurement results e.g. on monitors and/or converted into control signals for other field devices, such as e.g. magnetically operated valves, electric motors, etc., embodied as actuators for process control. Accordingly, the data processing system serves usually also for conditioning the measured-value signal delivered from the measuring electronics corresponding to the requirements of downstream data transmission networks, for example suitably digitizing such and, on occasion, converting it into a corresponding telegram, and/or evaluating it on-site. For such purposes, provided in these data processing systems, electrically coupled with the pertinent connection lines, are evaluating circuits, which pre- or further-process, and, if required, suitably convert, measured-values received from the measuring electronics. Serving for data transmission in such industrial data processing systems, as least sectionally, are, especially serial, fieldbuses, such as e.g. FOUNDATION FIELDBUS, CAN, CAN-OPEN, RACKBUS-RS 485, PROFIBUS, etc. or, for example, also networks based on the ETHERNET standard, as well as the corresponding standardized transmission protocols, which are, most often, independent of application.
Usually, it is possible to implement by means of control computers, besides such process visualization, monitoring and control, also remote servicing, parametering and/or monitoring of the connected measuring system. Accordingly, measuring electronics of modern, measuring, field devices permit, besides actual measured-value transmission, also transmission of various setting- and/or operating-parameters used in the measuring system, such as e.g. calibration data, measured-value ranges and/or also diagnostic values ascertained internally in the field devices. In support of this, operating data intended for the measuring system can, most often, likewise be sent via the aforementioned data transmission networks, which are, most often, hybrid as regards transmission physics and/or transmission logic.
Besides the evaluating circuits required for processing and converting measured-values delivered from connected measuring electronics, superordinated data processing systems of the described kind include, most often, also electrical supply circuits serving for supplying the connected measuring electronics and, as a result, also the pertinent measuring system with electrical energy, or power. The supply circuits provide for the pertinent measuring device electronics an appropriate supply voltage, which is, on occasion, fed directly by the connected fieldbus, and drive the electrical lines connected to the measuring device electronics, as well as the electrical currents flowing therethrough. A supply circuit can, in such case, for example, be assigned to exactly one measuring electronics and accommodated together with the evaluating circuit associated with the particular measuring device, for example joined to form a corresponding fieldbus adapter, in a housing common to both, embodied e.g. as a top-hat rail module. It is, however, also quite usual to accommodate such superordinated evaluating circuits and supply circuits, in each case, in separate housings, on occasion spatially removed from one another and to wire them appropriately together via external cables.
In the case of industrial measuring systems of the type being discussed here, often involved, as a result, are spatially distributed measuring systems, wherein, in each case, a plurality of measured variables of equal and/or different type are locally registered by sensors at real, mutually separated measuring points along a flow axis of the measuring system defined by the process line. These measured variables are fed to the common measuring electronics in the form of corresponding, electrical, measurement signals by wire, for example also in the so-called HART®-MULTIDROP-method or also in the so-called burst-mode method, and/or wirelessly, especially by radio and/or optically, on occasion also encoded into a digital signal or in a digitally transmitted telegram. For the case described above, in which such a measuring system is formed by means of a flow transducer, it is thus possible, for example in addition to the at least one, practically directly registered, flow parameter serving as primary measured variable, for example the volume flow, to ascertain, at least indirectly and, as a result, to measure, by means of the same measuring electronics, at least virtually, with application also of other, remotely registered, measured variables, for example, a remote, local temperature or a remote, local pressure in the medium, also derived, secondary measured variables, such as e.g. a mass flow and/or a density.
Experimental investigations on distributed measuring systems of the type being discussed, which, as shown e.g. also in U.S. Pat. No. 6,651,512, ascertain, by means of a directly measured, volume flow and a virtually measured density, a mass flow as an indirectly measured variable, have shown that, especially also despite application of internally, as well as externally, ascertained, measured variables proved to be very precise in the measuring ranges usual for the pertinent caliber of the process line, significant errors can arise in the result of a measurement virtual in the above sense. It is quite possible for these errors to lie in the range of about 5% of the actual measured variable or even beyond. This is the case, especially also when ascertaining measured variables, such as e.g. volume flow, temperature or pressure, as intermediate, really measured variables, and/or density as an intermediate variable measured virtually according to measuring and calculating methods recommended in the aforementioned industrial standards.
Further, comparative investigations have, in such case, additionally shown that the aforementioned measurement errors can show, among other things, a certain dependence on the instantaneous Reynolds number of the flow, as well as also on the instantaneous thermodynamic state of the medium. However, it has also been found, in this connection, that, in numerous industrial applications, especially those involving compressible and/or at least 2-phase media, the Reynolds number, or the thermodynamic state of the medium, can be not only chronologically but also spatially variable to a high degree, especially in the direction of the flow axis of the measuring system. Besides applications having at least partially compressible media, additionally especially also applications show a significant transverse sensitivity to spatial variances of the Reynolds number, or the thermodynamic state, when the measurement of at least one of the measured variables occurs at a measuring point (real or virtual), at which the process line has a caliber deviating at least from one of the measuring points (real or virtual) to the other. This is e.g. the case in the application of flow conditioners reducing the cross section of the line (such as in the case of e.g. nozzles serving as so-called reducers), which can find application in the inlet region of flow measuring transducers, or also in the application of flow conditioners increasing the cross section of the line (so-called diffusers) in the outlet region of flow measuring transducers. Measuring systems with such reducers and/or diffusers are described, for example, in GB-A 21 42 725, U.S. Pat. No. 5,808,209, US-A 2005/0092101, U.S. Pat. Nos. 6,880,410, 6,644,132, 6,053,054, 6,644,132, 5,052,229 or 6,513,393 and are used, for example, for improving accuracy of measurement of flow measuring transducers. It has, in such case, been further ascertained that such transverse sensitivities based on application of reducers and/or diffusers are significant for caliber ratios between about 0.6 and 0.7, while their influence for caliber ratios with extreme diameter jumps of smaller than 0.2 are quite negligible.
Another application area having a significant sensitivity to the aforementioned variances as affecting the desired accuracy of measurement concerns, furthermore, those measuring systems, which are provided for the flow measurement of heavy gases, such as, perhaps, carbon dioxide or also phosgene, or long-chain carbon compounds having a molecular wa of over 30 g/mol.
The above-described spatial variance of the Reynolds number can, in turn, lead to the fact that practically each of the aforementioned, mutually spaced, real measuring points of the distributed measuring system has, during operation, a local Reynolds number deviating, to a considerable degree, from the local Reynolds number of each of the other, also-used, measuring points. Equally, also the mentioned variance of the thermodynamic state would lead to the fact that mutually spaced, measuring points of the distributed measuring system can have thermodynamic states differing from one another. In view of this, thus, each of the measured variables, as measured on a distributed basis, would have to be adjusted according to the particularly associated, local Reynolds number and/or the particularly associated, local thermodynamic state, a task which, in the absence of the information required therefor, namely the, in each case, other, but remotely measured, state variables, is not directly possible. If, for example, the density and/or the mass flow, calculated on the basis of the measured state variables pressure and temperature, would be calculated without taking into consideration the variance of the Reynolds number, or thermodynamic state, an additional measurement error would result, having essentially a quadratic dependence on the flow velocity. Accordingly, for the aforementioned configuration, at flow velocities of clearly less than 10 m/s, the measuring accuracy of about 0.1% to 0.5%, currently strived for, is practically no longer significant.
Starting from the above-described disadvantages of measuring systems of the described kind, especially those ascertaining a mass flow or a volume flow, an object of the invention is to increase the accuracy of measurement for such secondary measured variables ascertained with application of spatially, distributedly registered, thermodynamic state variables such as pressure and/or temperature.
For achieving the object, the invention resides in a measuring system for measuring a density of a medium, which is variable as regards a thermodynamic state, especially at least partially compressible, flowing in a process line along a flow axis of the measuring system. The measuring system includes therefor: At least one temperature sensor placed at a temperature measuring point, reacting primarily to a local temperature, Θ, of medium flowing past, and delivering at least one temperature measurement signal influenced by the local temperature of the medium to be measured; at least one pressure sensor placed at a pressure measuring point, reacting primarily to a local pressure, p, especially a static pressure, of medium flowing past, and delivering at least one pressure measurement signal influenced by the local pressure, p, in the medium to be measured; and a measuring electronics communicating, in each case, at least at times, with at least the temperature sensor and the pressure sensor, and producing, at least at times, with application both of the temperature measurement signal and also at least the pressure measurement signal, at least one density measured-value, especially a digital density measured-value representing, instantaneously, a local density, ρ, of the flowing medium at a virtual, density measuring point, especially a locationally fixed, virtual, density measuring point, predeterminably spaced from the pressure measuring point and/or the temperature measuring point along the flow axis.
In a first embodiment of the invention, it is provided that the measuring electronics includes a data memory, especially a non-volatile data memory, which stores, at least at times, at least one measuring system parameter specifying solely the medium currently to be measured, especially a system parameter such as a specific heat capacity, cp, of the medium currently to be measured, a molar mass, n, of the medium and/or the number, f, of degrees of oscillatory freedom of the atoms, or molecules, of the medium, as determined by the molecular structure of the medium.
In a second embodiment of the invention, it is provided that the measuring electronics ascertains the density measured-value with application of the at least one measuring system parameter specifying solely the medium currently to be measured.
In a third embodiment of the invention, it is provided that the measuring electronics includes a data memory, especially a non-volatile data memory, which stores, at least at times, at least one measuring system parameter specifying both the medium to be measured by means of the measuring system as well as also instantaneous circumstances of installation of the measuring system, wherein the circumstances of installation are determined by the arrangement of pressure-, temperature- and density measuring points relative to one another, as well as, in each case, by the form and size of the process line in the areas of the pressure-, temperature- and density measuring points. In a further development of this embodiment of the invention, the measuring electronics ascertains the density measured-value with application of the at least one measuring system parameter specifying both the medium currently to be measured by means of the measuring system as well as also instantaneous circumstances of installation of the measuring system.
In a fourth embodiment of the invention, it is provided that the measuring electronics includes a data memory, especially a non-volatile data memory, which stores, at least at times, at least one measuring system parameter of a first kind specifying the medium currently to be measured, especially a specific heat capacity of the medium currently to be measured, a molar mass of the medium and/or the number of degrees of freedom of the medium, and which stores, at least at times, at least one measuring system parameter of a second kind specifying both the medium currently to be measured as well as also instantaneous circumstances of installation of the measuring system, wherein the instantaneous circumstances of installation are determined by the arrangement of pressure-, temperature- and density-measuring points relative to one another, as well as, in each case, by the form and size of the process line in the regions of the pressure-, density- and/or temperature-measuring points, and wherein the measuring electronics ascertains the density measured-value with application at least of the measuring system parameter of the first kind and the measuring system parameter of the second kind.
In a fifth embodiment of the invention, it is provided that the measuring electronics receives, at least at times, numerical parameter values, especially numerical parameter values ascertained, externally of the measuring system and/or near in time, for at least one measuring system parameter specifying a medium to be measured and/or instantaneous circumstances of installation of the measuring system, especially a heat capacity, cp, for medium to be measured, which represents a specific heat capacity, cp, earlier ascertained and/or measured remotely from the density measuring point for the medium to be measured.
In a sixth embodiment of the invention, it is provided that the measuring electronics communicates, especially via fieldbus, at least at times, especially by wire and/or by radio, with a superordinated, electronic, data processing system. In a further development of this embodiment of the invention, it is additionally provided that the measuring electronics transmits the density measured-value to the data processing system and/or wherein the measuring electronics receives from the data processing system, at least at times, measuring system parameters specifying numerical parameter values for the medium to be measured currently, especially its thermodynamic properties and/or its chemical composition, especially a specific heat capacity, cp, of the medium currently to be measured, a molar mass, n, of the currently to be measured medium and/or the number, f, of degrees of oscillatory freedom of the atoms, or molecules, of the currently to be measured medium, and/or that the measuring electronics is connected with the superordinated, electronic, data processing system by means of a fieldbus, especially a serial fieldbus.
In a seventh embodiment of the invention, it is provided that the measuring electronics ascertains, during operation, at least at times, a specific heat capacity, cp, of the currently to be measured medium, especially on the basis of the formula:
            c      P        =                  (                  1          +                      f            2                          )            ·              R        n              ,wherein n is a molar mass, R, the absolute gas constant, with R=8.3143 J/(K mol) and f, a number, determined by the molecular structure of the medium, of degrees of oscillatory freedom of its atoms, or molecules.
In an eighth embodiment of the invention, it is provided that the measuring electronics generates, repetitively, a temperature measured-value, especially a digital temperature measured-value, based on the temperature measurement signal, and wherein the temperature measured-value represents, instantaneously, the temperature of the medium at the temperature measuring point.
In a ninth embodiment of the invention, it is provided that the measuring electronics generates, repetitively, a pressure measured-value, especially a digital pressure measured-value, based on the pressure measurement signal, and wherein the pressure measured-value represents a pressure instantaneously reigning in the medium, especially at the pressure measuring point.
In a tenth embodiment of the invention, it is provided that the measuring system further includes a flow sensor placed at a flow measuring point and reacting, primarily, to a local flow parameter, especially a flow parameter averaged over a cross section of the process line, especially a flow velocity, a volume flow or a mass flow, of the medium to be measured, especially also changes of the same, and wherein the flow sensor delivers at least one flow measurement signal influenced by the local flow parameter.
Developing this embodiment of the invention further, it is provided that                the measuring electronics communicates, at least at times, also with the flow sensor, and wherein the measuring electronics ascertains the density measured-value with application also of the flow measurement signal; and/or        the medium has, at the virtual density measuring point, a thermodynamic state corresponding to a thermodynamic state of the medium at the velocity measuring point; and/or        the virtual density measuring point and the flow measuring point at least partially overlap one another, especially they are coincident; and/or        the temperature measuring point and the flow measuring point at least partially overlap one another, especially they are coincident; and/or        the pressure measuring point and the flow measuring point at least partially overlap one another; and/or        the density measured-value represents a local density of the medium in the region of the flow sensor; and/or        the measuring electronics communicates with the flow sensor by means of a field bus, especially a serial field bus, and/or wirelessly by radio; and/or        the measuring electronics communicates, at least at times, with the flow sensor, wherein the measuring electronics ascertains, with application at least of the flow measurement signal, a velocity measured-value, especially a digital flow measured-value, which represents instantaneously the flow velocity of the flowing medium.        
In an eleventh embodiment of the invention, it is provided that the measuring electronics produces the density measured-value also with application of at least one numerical compensation factor, especially a digitally stored compensation factor, which corresponds with a locational variability occurring along the flow axis of the measuring system, especially a locational variability ascertained in advance or during operation, of at least one thermodynamic state variable of the medium, especially a temperature, a pressure or a density, and/or with a locational variability occurring along the flow axis of the measuring system, especially a locational variability ascertained in advance or during operation, of the Reynolds number of the flowing medium.
Developing this embodiment of the invention further, it is additionally provided that                the at least one compensation factor is ascertained taking into consideration the medium actually to be measured, especially its composition and/or its thermodynamic properties, especially during a calibration of the measuring system with known, reference medium and/or during start-up of the measuring system on-site; and/or        the measuring electronics ascertains a compensation factor, at least once, during start-up of the measuring system; and/or        the measuring electronics repetitively ascertains the compensation factor during operation of the measuring system, especially in conjunction with a change of at least one chemical property of the medium to be measured or with a replacement of the same with another medium; and/or        the measuring electronics ascertains the at least one compensation factor on the basis of a predetermined, specific heat capacity, cp, of the current medium, especially a heat capacity ascertained in dialog with a user and/or externally of the measuring electronics; and/or        the measuring electronics includes a data memory storing the at least one compensation factor, especially a data memory embodied as a table memory and/or a non-volatile memory; and/or        the data memory stores a plurality of compensation factors ascertained in advance for different media and/or for different circumstances of installation; and/or        the measuring electronics selects the at least one compensation factor taking into consideration the current medium, as well as the current circumstances of installation, from the plurality of compensation factors stored in the data memory.        
In a twelfth embodiment of the invention, it is provided that the measuring electronics produces the density measured-value with application of at least one density correction value ascertained at run time, dependent both on a flow velocity of the medium as well as also on the local temperature reigning at the temperature measuring point, wherein the correction value corresponds with an instantaneous, locational variability of at least one thermodynamic state variable of the medium, especially with such an instantaneous, locational variability related to the medium currently to be measured as well as to instantaneous circumstances of installation and/or with such an instantaneous, locational variability occurring along the flow axis of the measuring system, and/or wherein the correction value corresponds with an instantaneous locational variability of the Reynolds number of the flowing medium, especially with a locational variability of the Reynolds number related to the medium and/or the type of construction of the measuring system, or with an instantaneous variability of the Reynolds number occurring along the flow axis of the measuring system.
Further developing this embodiment of the invention, it is further provided that                the measuring electronics ascertains, during operation, a velocity measured-value, especially a digital velocity measured-value, representing, instantaneously, the flow velocity of the flowing medium and that the measuring electronics ascertains, with application of the velocity measured-value as well as the temperature measured-value, the density correction value; and/or        the measuring electronics compares, repetitively, during operation, the density correction value with at least one predetermined reference value; and/or        the measuring electronics, based on a comparison of density correction value and reference value, quantitatively signals an instantaneous deviation of the density correction value from the reference value and/or generates an alarm, which signals an undesired discrepancy, especially an unallowably high discrepancy, between density correction value and associated reference value.        
In a thirteenth embodiment of the invention, it is provided that the measuring electronics, based on the pressure measurement signal, as well as on the temperature measurement signal, ascertains a provisional density measured-value, especially according to one of the industry standards AGA 8, AGA NX-19, SGERG-88 IAWPS-IF97, ISO 12213:2006, representing a density which the flowing medium only apparently has at the virtual density measuring point.
Further developing this embodiment of the invention, it is additionally provided that                the measuring electronics ascertains, repetitively during operation, a density error corresponding with a deviation, especially a relative deviation, of the provisional density measured-value from the density measured-value, and especially issues such also in the form of a numerical, density measured-value; and/or        the measuring electronics issues an instantaneous density error corresponding with a deviation, especially a relative deviation, of provisional density measured-value and density measured-value, in the form of a numerical, density error value and/or compares the instantaneous density error with at least one predetermined reference value and, based on this comparison, generates, at times, an alarm signaling an undesired, especially impermissibly high, discrepancy between provisional density measured-value and density measured-value.        
In a fourteenth embodiment of the invention, it is provided that the measuring system includes, further, at least one flow sensor placed at a flow measuring point and reacting primarily to a local flow parameter, especially a flow parameter averaged over a cross section of the process line, especially a flow velocity, a volume flow or a mass flow of the medium to be measured, especially also to changes of the same, and delivering at least one flow measurement signal influenced by the local flow parameter, wherein                the measuring electronics communicates, at least at times, with the flow sensor and wherein the measuring electronics, with application at least of the flow measurement signal, ascertains a volume flow measured-value, especially a digital volume flow measured-value, representing, instantaneously, a volume flow rate of the flowing medium; and/or        the measuring electronics ascertains, with application at least of the density measured-value and the volume flow measured-value, a mass flow measured-value, especially a digital mass flow measured-value, representing, instantaneously, a mass flow rate of the flowing medium; and/orwherein the measuring electronics ascertains, with application at least of the temperature measurement signal, the pressure measurement signal and the flow measurement signal, a mass flow measured-value, especially a digital mass flow measured-value, representing, instantaneously, a mass flow rate of the flowing medium; and/orthe flow measuring point is arranged upstream of the temperature measuring point and/or upstream of the pressure measuring point; and/or        the at least one flow sensor is formed by means of at least one piezoelectric element and/or by means of at least one piezoresistive element; and/or        the at least one flow sensor is formed by means at least of an electrical resistance element, especially a resistance element through which a heating current flows, at least at times; and/or        the at least one flow sensor is formed by means of at least one measuring electrode tapping electrical potentials, especially a measuring electrode contacting flowing medium; and/or        the at least one flow sensor is formed by means of at least one measuring capacitor reacting to changes of the flow parameter; and/or        the at least one flow sensor, especially a flow sensor protruding at least partially into a lumen of the process line, is located downstream of at least one bluff body immersed in the medium and protruding into a lumen of the process line.        
In a fifteenth embodiment of the invention, it is provided that the measuring electronics communicates with the temperature sensor by means of a fieldbus, especially a serial fieldbus, and/or wirelessly by radio.
In a sixteenth embodiment of the invention, it is provided that the measuring electronics communicates with the pressure sensor by means of a field bus, especially a serial fieldbus, and/or wirelessly by radio.
In a seventeenth embodiment of the invention, it is provided that the medium at the density measuring point is in a thermodynamic state differing, at least at times, significantly, especially to a degree significant for a desired accuracy of the measuring accuracy of the measuring system, as regards at least one local, thermodynamic state variable, especially a temperature and/or a pressure and/or a density, from a thermodynamic state of the medium at the temperature measuring point and/or a thermodynamic state of the medium at the pressure measuring point.
In an eighteenth embodiment of the invention, it is provided that the flowing medium has a Reynolds number greater than 1000.
In a nineteenth embodiment of the invention, it is provided that the medium is compressible, having, especially, a compressibility K=−1/V·dV/dp, which is greater than 10−6 bar−1, and/or is at least partially gaseous. The medium can, in such case, be a gas loaded with solid particles and/or with droplets.
In a twentieth embodiment of the invention, it is provided that the medium has two or more phases. One phase of the medium can, in such case, be liquid and/or the medium can be a liquid containing gas and/or solid particles.
In a twenty-first embodiment of the invention, it is provided that the measuring system further includes a display element communicating, at least at times, with the measuring electronics, for visual signalizing at least of the density measured-value.
In a twenty-second embodiment of the invention, it is provided that the process line is embodied, at least sectionally, especially in the region at least of the density measuring point and/or in the region at least of the pressure measuring point, as a pipeline essentially stable in form at least under operating pressure, especially in the form of a rigid pipeline and/or a pipeline having a circular cross section.
In a twenty-third embodiment of the invention, it is provided that the process line is embodied at least sectionally, especially in the region between density measuring point and pressure measuring point and/or between density measuring point and temperature measuring point, as an essentially straight pipeline, especially a pipeline having a circular cross section.
In a twenty-fourth embodiment of the invention, it is provided that the process line has at the virtual density measuring point a caliber differing from a caliber of the process line at the pressure measuring point. Developing this embodiment of the invention further, it is provided that the caliber of the process line is greater at the pressure measuring point than the caliber of the process line at the virtual density measuring point, especially it is provided that a caliber ratio of the caliber of the process line at the pressure measuring point to the caliber of the process line at the virtual density measuring point is kept greater than 1.1.
In a twenty-fifth embodiment of the invention, it is provided that a caliber ratio of a caliber of the process line at the pressure measuring point to a caliber of the process line at the virtual density measuring point is kept smaller than 5.
In a twenty-sixth embodiment of the invention, it is provided that a caliber ratio of a caliber of the process line at the pressure measuring point to a caliber of the process line at the virtual density measuring point is kept in a range between 1.2 and 3.1.
In a twenty-seventh embodiment of the invention, it is provided that the process line has, between the virtual density measuring point and the pressure measuring point, a line segment which is embodied as a diffuser, especially a funnel-shaped diffuser, having a lumen widening in the flow direction, especially continuously widening.
In a twenty-eighth embodiment of the invention, it is provided that the process line has, between the virtual density measuring point and the pressure measuring point, a line segment which is embodied as a nozzle, especially a funnel-shaped nozzle, having a lumen narrowing in the flow direction, especially continuously narrowing.
In a twenty-ninth embodiment of the invention, it is provided that the process line has at the virtual density measuring point a caliber which is essentially equal to a caliber of the process line at the pressure measuring point.
In a thirtieth embodiment of the invention, it is provided that the process line has, at the virtual density measuring point, a caliber differing from a caliber of the process line at the temperature measuring point. Developing this embodiment of the invention further, it is additionally provided that the caliber of the process line is greater at the temperature measuring point than the caliber at the virtual density measuring point, especially that a caliber ratio of the caliber of the process line at the temperature measuring point to the caliber of the process line at the virtual density measuring point is kept greater than 1.1.
In a thirty-first embodiment of the invention, it is provided that a caliber ratio of the caliber of the process line at the temperature measuring point to the caliber of the process line at the virtual density measuring point is kept smaller than 5.
In a thirty-second embodiment of the invention, it is provided that a caliber ratio of the caliber of the process line at the temperature measuring point to the caliber of the process line at the virtual density measuring point is kept in a range between 1.2 and 3.1.
In a thirty-third embodiment of the invention, it is provided that the process line has, between the virtual density measuring point and the temperature measuring point, a line segment embodied as a diffuser, especially a funnel-shaped diffuser, having a lumen widening in the flow direction, especially continuously widening.
In a thirty-fourth embodiment of the invention, it is provided that the process line has, between the virtual density measuring point and the temperature measuring point, a line segment embodied as a nozzle, especially a funnel-shaped nozzle, having a lumen becoming narrower in the flow direction, especially continuously narrower.
In a thirty-fifth embodiment of the invention, it is provided that the process line has, at the virtual density measuring point, a caliber essentially equal to a caliber of the process line at the temperature measuring point.
In a thirty-sixth embodiment of the invention, it is provided that the virtual density measuring point is placed upstream of the temperature measuring point and/or upstream of the pressure measuring point.
In a thirty-seventh embodiment of the invention, it is provided that the pressure measuring point is arranged downstream from the temperature measuring point.
In a thirty-eighth embodiment of the invention, it is provided that a separation of the pressure measuring point from the virtual density measuring point is different from a separation of the temperature measuring point from the virtual density measuring point.
In a thirty-ninth embodiment of the invention, it is provided that a separation of the pressure measuring point from the virtual density measuring point is greater than a separation of the temperature measuring point from the virtual density measuring point.
In a fortieth embodiment of the invention, it is provided that a separation of the pressure measuring point from the virtual density measuring point is greater than a caliber of the process line at the pressure measuring point and/or wherein a separation of the pressure measuring point from the temperature measuring point is greater than a caliber of the process line at the pressure measuring point.
Developing this embodiment of the invention further, it is additionally provided that a separation of the pressure measuring point from the virtual density measuring point corresponds at least to 3-times, especially more than 5-times, a caliber of the process line at the pressure measuring point and/or that a separation of the pressure measuring point from the temperature measuring point corresponds at least to 3-times, especially more than 5-times, a caliber of the process line at the pressure measuring point.
In a forty-first embodiment of the invention, it is provided that the measuring electronics includes a microcomputer. Developing this embodiment of the invention further, it is additionally provided that the measuring electronics produces at least the density measured-value by means of the microcomputer.
In a forty-second embodiment of the invention, it is provided that the measuring system further includes at least one electronics housing, especially an explosion- and/or pressure- and/or impact- and/or weather-resistant housing, in which the measuring electronics is at least partially accommodated. In a further development of this embodiment, it is additionally provided that the at least one, especially metal, electronics housing is held to the process line and/or placed in the immediate vicinity of the virtual density measuring point.
A basic idea of the invention is to improve accuracy of measurement of measuring systems of the described kind by ascertaining, with improved accuracy, the density derived from, indeed, real, but, however, of necessity, distributedly measured, state variables. This derived density serves as a central measured variable in numerous applications of industrial measurements technology in the case of flowing media. The improved accuracy is achieved by taking into consideration possible spatial variance, especially also the degree thereof, of Reynolds number and/or thermodynamic state of the flowing medium. This is done in the case of the measuring system of the invention by a reliable calculating of the density, by referencing it to a reference point defined earlier for the particular measuring system and serving as a locationally fixed, imaginary, measuring point. The density is, thus, measured virtually. Developing this basic idea further, the measurement accuracy, with which the measuring system ascertains the local density, can be significantly improved further by having the measuring system ascertain said density also taking into consideration an equally locally measured, extant flow velocity, in order to achieve a further compensation of the error accompanying the mentioned variances of Reynolds number and/or thermodynamic state of the flowing medium.
The invention is based, in such case, on the surprising discovery that spatial variance in the Reynolds number and/or in the thermodynamic state, and the measurement errors associated therewith, can be projected onto a single dimension lying in the flow direction and/or coinciding with the flow axis of the measuring system and, thus, can be mapped into a correspondingly simplified set of measuring system parameters, which can be ascertained, at least predominantly in advance, experimentally and/or with computer support, for example in the course of a calibration of the measuring system, during completion of manufacturing and/or during start-up of the same. The spatial variances, or their extent and, as a result, also the set of device parameters, are, it is true, specific for each concrete measuring system and each concrete medium, so that the calibration is individual, but such can then, however, be viewed as invariant in the face of possible changes of Reynolds number and/or thermodynamic state arising during operation, if the measuring system remains unchanged, with essentially constant medium as regards its chemical composition. In other words, for a given, distributed measuring system, the size of changes of the thermodynamic state arising along the flow axis can be determined ahead of time, so that their influence can be calibrated and, as a result, also compensated with accuracy sufficient for the measurements, with it having been found, surprisingly, that the size of the change is largely constant for a given measuring system with constant medium, so that such can be mapped into a set of, it is true, specific, but also constant, device parameters.
An advantage of the invention is additionally to be seen in the fact that the fundamental method can be directly retrofitted into numerous, already installed, measuring systems, at least insofar as the measuring device electronics permits a change of the pertinent processing software.